Method and a device for detecting an abnormality of a heat exchanger and the use of such a device

ABSTRACT

A method and a device for detecting an abnormality of a heat exchanger exchanging heat between a first fluid flow flowing in a conduit and a second fluid flow flowing along a flow path, said conduit and said flow path each having an inlet and an outlet, whereby the method comprises the steps of establishing at least one parameter representative of the temperature conditions of the heat exchanger, establishing a second fluid inlet temperature, establishing a parameter indicative of expected heat exchange between the heat exchanger and the second fluid, processing the heat exchanger temperature, the second fluid temperature and the parameter indicative of expected heat exchange for establishing an estimated second fluid outlet temperature, and employing the estimated second fluid outlet temperature for evaluating the heat exchange between the first and second fluids by comparing the estimated second fluid outlet temperature, or a parameter derived therefrom, with a reference value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates byreference essential subject matter disclosed in international PatentApplication No. PCT/DK2003/000701 filed on Oct. 14, 2003 and DanishPatent Application No. PA 2002 01582 filed on Oct. 15, 2002.

FIELD OF THE INVENTION

The present invention relates to a method and a device for detecting anabnormality of a heat exchanger exchanging heat between a first fluidflow flowing in a conduit and a second fluid flow flowing along a flowpath, said conduit and said flow path each having an inlet and anoutlet, and the use of such a device.

BACKGROUND OF THE INVENTION

Heat exchangers are an important part of many plants and systems,especially refrigeration or heat pump systems. These heat exchangers andtheir efficiency are of crucial importance in such systems, and it istherefore important to monitor functioning of the heat exchangers to beable to detect abnormality of the heat exchanger, so measures can betaken to remedy any defects.

By abnormality of the heat exchanger is meant that the heat exchangerdoes not exchange as much energy as expected, i.e. the fluids do notexperience the cooling or heating they should. This may be due tofouling of heat exchanger, in that a layer of scale, dirt or grease isdeposited on the heat exchanging surface or surfaces leading to reducedheat exchange, as this layer will usually act as an insulating layer.Another possibility is that there is insufficient fluid flow because ofdirt or the like blocking or restricting flow through the heatexchanger. Both situations lead to higher power consumption, because thesystem must work at a higher load than a system working with heatexchangers within the normal range. Further in the event of adverseworking conditions with high heat exchange demand and a relatively smalltemperature difference between the fluids, it may be impossible to meetthe demand, which in some systems may have devastating effect.

Often the abnormality will not be detected before an adverse workingcondition is experienced, in that the demand cannot be met, e.g. leadingto an increase of the temperature of a system, which should be kept at aspecific temperature. An example of such a system is a refrigerateddisplay cabinet in a shop, where strict legislation in most countriesprescribes that when food is not kept below a maximum temperature, itmust be discarded, which of course is expensive and devastating for thebusiness. Likewise large computer systems are often kept inair-conditioned rooms, as an excessive temperature may increase the riskof a computer crash, which may entail a high risk of data loss and lostman-hours.

Common provisions for detecting abnormality of a heat exchanger includebasic visual inspection at regular intervals to check for dirt at theinlet of the heat exchanger. Often the heat exchangers are placed soinspection is difficult, and hence such inspection is labour consuming.Further an abnormality may arise at different intervals and quitequickly, e.g. in the event of material blocking the inlet to the heatexchanger. This means that to provide a reasonable degree of securityagainst heat exchanger abnormality, it is necessary to inspect the heatexchangers often. Further a visual inspection of the outside of the heatexchanger may not be effective in assessing whether the internal heatexchanging surfaces are subject to fouling etc. causing a reduced heatexchange.

Another known way of detecting abnormality of a heat exchanger is bydirect flow measurement. A direct flow measurement requires delicate andexpensive equipment, such as hot wire anemometers or the like, and aplurality of flow measurement devices should be used to gain usefulinformation on the overall flow field. It has also been proposed toassess the flow based on pressure sensors, but such pressure sensors arealso expensive, and to gain useful information on the overall flow fielda plurality of pressure sensors should be used. A further disadvantageof these methods are that they can only be used to establish if there isrestricted flow in the heat exchanger, not the situation where the flowis normal, but heat exchange is reduced, e.g. because of fouling of theheat exchanger surface.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method to enableearly detection of an abnormality of a heat exchanger.

This object is met by a method comprising the steps of:

-   -   establishing at least one parameter representative of the        temperature conditions of the heat exchanger,    -   establishing a second fluid inlet temperature,    -   establishing a parameter indicative of expected heat exchange        between the heat exchanger and the second fluid,    -   processing the heat exchanger temperature, the second fluid        temperature and the parameter indicative of expected heat        exchange for establishing an estimated second fluid outlet        temperature,    -   employing the estimated second fluid outlet temperature for        evaluating the heat exchange between the first and second fluids        by comparing the estimated second fluid outlet temperature, or a        parameter derived therefrom, with a reference value. By this        method is provided a convenient way of assessing the functioning        of the heat exchanger primarily based on parameters of the first        fluid, which means that a minimum of sensors are needed for        providing information regarding the second fluid, and an        automatic detection of heat exchanger abnormality is made        possible. Further this method makes it possible to detect        restricted flow as well as fouling of the heat exchanger.

According to an embodiment, the reference value is a predeterminedsecond fluid outlet temperature.

An even more reliable, alternative method can be obtained when themethod comprises the steps of using the estimated second fluid outlettemperature for establishing a second heat rate of the second fluid forevaluating the energy balance of the second heat rate of the secondfluid compared to a first heat rate of the first fluid, as an evaluationbased on the assumption of energy balance will take the influence offurther parameters into account.

According to an embodiment, the method comprises establishing the secondrate of heat flow of the second fluid by establishing an estimate of asecond fluid mass flow and a specific enthalpy change of the secondfluid across the heat exchanger based on the estimated second fluidoutlet temperature and the second fluid inlet temperature.

According to an embodiment, the method comprises establishing the firstrate of heat flow by establishing a first fluid mass flow and a specificenthalpy change of the first fluid across the heat exchanger based onparameters representative for first fluid inlet and outlet temperatures,and the condensation pressure.

A direct evaluation of the heat exchange is possible, but may however besubject to some disadvantages, e.g. because of fluctuations orvariations of the parameters in the refrigeration or heat pump system,and according to an embodiment, the method comprises establishing aresidual as difference between the first heat rate and the second heatrate.

It may also be possible to evaluate the heat exchange by directevaluation of the estimated outlet second fluid outlet temperature, butthis may however be subject to some disadvantages, e.g. because offluctuations or variations of the parameters in the refrigeration orheat pump system, and according to an alternative embodiment, the methodcomprises establishing a residual as difference between the estimatedand predetermined second fluid outlet temperature.

To further reduce the sensibility to fluctuations or variations ofparameters in the system and be able to register a trend of heatexchanging, the method comprises providing an abnormality indicator bymeans of the residual, the abnormality indicator being providedaccording to the formula: $S_{\mu,i} = \left\{ \begin{matrix}{{S_{\mu,{i - 1}} + s_{i}},{{{{when}\quad S_{\mu,{i - 1}}} + s_{\mu,i}} > 0}} \\{0,{{{{when}\quad S_{\mu,{i - 1}}} + s_{\mu,i}} \leq 0}}\end{matrix} \right.$where s_(μ,i) is calculated according to the following equation:$s_{\mu,i} = {c_{1}\left( {r_{i} - \frac{\mu_{0} + \mu}{2}} \right)}$where c₁ is a proportionality constant, μ₀ a first sensibility value,and u a second sensibility value.

Another aspect of the invention regards a heat exchanger abnormalitydetection device for a heat exchanger exchanging heat between a firstfluid in a conduit and a second fluid in a flow path, where the devicecomprises a first estimator estimating a heat exchanger temperature, afirst intermediate memory means storing the heat exchanger temperature,a temperature sensor measuring the second fluid inlet temperature, asecond intermediate memory means storing the second fluid inlettemperature, a second estimator establishing a parameter indicative ofexpected heat exchange between the heat exchanger and the second fluid,a third intermediate memory means storing the parameter indicative ofexpected heat exchange, a processor establishing an estimated secondfluid outlet temperature based on said heat exchanger temperature, saidsecond fluid inlet temperature, from the first and second intermediatememory means, respectively, and, from the third intermediate memorymeans, the parameter indicative of expected heat exchange, and acomparator comparing the estimated second fluid outlet temperature, or aparameter established on basis thereof, with a reference value.

An embodiment of the device further comprises memory means for storingat least one parameter from the processor, whereby a device is obtainedwhich may operate on the basis of previously stored data.

Although applicable to heat exchangers in general, it is found that thedevice is particularly suited for an embodiment, where the heatexchanger is part of a vapour-compression refrigeration or heat pumpsystem comprising a compressor, a condenser, an expansion device, and anevaporator interconnected by conduits providing a flow circuit for thefirst fluid, said first fluid being a refrigerant.

According to an embodiment, the heat exchanger is the condenser, whichis particularly difficult to monitor, as the refrigerant in thecondenser is present in three different phases, namely as superheatedgas, a mixture of gas and liquid and sub cooled liquid.

According to an embodiment, the second fluid is air, which is the mostcommon type of second fluid for refrigeration or heat pump systems asoutlined above, and for which direct measurement of fluid parametersinvolves some special problems. Further the air used normally is theambient air, which may contain different kinds of pollution, which maydeposit on the heat exchanger.

Specifically the evaporator may be part of a refrigerated displaycabinet positioned within a building and the condenser is positionedoutside the building, which is a special example, where the deviceaccording to the invention may be of particular value.

A third aspect regards use of a detection device as outlined above,where the detection device is used for detecting fouling of the heatexchanger and/or detecting insufficient flow of the second fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in detail by way ofexample with reference to the drawing, where

FIG. 1 is a sketch of a refrigeration system,

FIG. 2 is a schematic sectional view of a heat exchanger,

FIG. 3 is a schematic end view of the heat exchanger,

FIG. 4 is an example of a temperature profile in the heat exchanger,

FIG. 5 is a schematic log p,h-diagram of a refrigerant,

FIG. 6 is a curve of estimated and measured outlet temperature of acondenser,

FIG. 7 is a curve of a residual,

FIG. 8 is a curve of an abnormality indicator, and

FIG. 9 is an enlarged portion of the curve according to FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following reference will be made to a heat exchanger in a simplerefrigeration system, although the principle is equally applicable to aheat exchanger in other heat exchanging systems, and as understood bythe skilled person, the invention is in no way restricted to arefrigeration system.

In FIG. 1 is shown a simple refrigeration system 1 comprising acompressor 2, a condenser 3, an expansion device 4 and an evaporator 5,which are connected by a conduit 6 in which a refrigerant iscirculating. In vapour-compression refrigeration or heat pump systemsthe refrigerant circulates in the system and undergoes phase change andpressure change. In the system 1 a refrigerant gas is compressed in thecompressor 2 to achieve a high pressure refrigerant gas, the refrigerantgas is fed to the condenser 3 (heat exchanger), where the refrigerantgas is cooled and condensates, so the refrigerant is in liquid state atthe exit from the condenser 3, expanding the refrigerant in theexpansion device 4 to a low pressure and evaporating the refrigerant inthe evaporator 5 (heat exchanger) to achieve a low pressure refrigerantgas, which can be fed to the compressor 2 to continue the process.

A specific example for the heat exchanger is a condenser 3 of arefrigeration system for a frozen food storage cabinet or a refrigerateddisplay cabinet for shops. In FIG. 2 can be seen a schematic sectionalview of a cross-flow heat exchanger with a first fluid flow 7 in aconduit 6 and a second fluid flow 8 in a flow path 9. Given the exampleof a condenser 3 of a refrigeration system 1 as mentioned above, thefirst fluid is a refrigerant and the second fluid would normally be air.The refrigerant enters the condenser 3 as a superheated gas, whichduring the passage of the condenser 3 is cooled by the air flowingaround and past the hot conduits 6 containing the refrigerant, so therefrigerant gas is cooled to condensation temperature, condensates andleaves the condenser 3 as sub cooled liquid. To obtain and maintain aflow of air through the condenser 3, the condenser 3 is normallyprovided with a fan (not shown), which can be running constantly, in anon-off mode or with a varying speed.

Typically a condenser 3 of such a system is placed outside the shop,often on the roof, because if it is placed inside, it would lead totemperature increase in the shop, and normally the outside temperatureis lower than the inside temperature. However placing the condenser 3outside has the drawback the condenser 3 may be exposed to clogging upor fouling because of dirt, grease, leaves, newspapers, etc restrictingthe air flow 8. or reducing heat transfer from the refrigerant to theair, and further the heat exchanger is difficult to reach and toinspect. The intervals of clogging up may be very irregular due toweather conditions, e.g. precipitation, wind direction etc., pollution,seasonal changes, such as leaf fall, which makes it difficult to provideproper inspection intervals.

FIG. 3 is a schematic end view of a heat exchanger, which in the givenexample is the condenser 3, the inlet of which is partly covered byleaves. This means that the air flow through the flow path 9 of thecondenser 3 is restricted, and hence that heat exchange is reduced. Tobe able to detect such an abnormality, an air outlet temperatureestimator is proposed using an air flow dependent thermo conductivity α.The value of the thermo conductivity is dependent on the given heatexchanger and can be established at start up of the heat exchanger. Itis found that the value of this parameter is not critical, and the valuemay be established based on empirical values or values supplied by themanufacturer of the heat exchanger. The thermo conductivity is flowdependent, and for a heat exchanger comprising a fan forcing air throughthe heat exchanger, the thermo conductivity α can be expressed asα=α₀ω^(0.8),  (1)where ω is the speed of the fan, and α₀ is the thermo conductivity atno-flow condition.

An estimate for the air temperature through the condenser 3 can bedefined using the thermo conductivity α, which for a constant airflowgivesdt _(air)(y)=α(t _(cond,surf)(y)−t _(air)(y))dy  (2)where y denotes the distance from the air inlet, and y is represented asa normalized parameter, i.e. is the distance relative to the totallength of the flow path, so the outlet is at 1, t_(cond,surf)(y) is thesurface temperature of the condenser heat exchanging surface at positiony, and t_(air)(y) is the air temperature at position y. t_(cond,surf)could be established by direct measurement using a temperature sensor.Such sensors, however, are expensive and especially when placed outsidethey are subject to errors. It is hence preferred to establish anestimate of the surface temperature of the condenser based on evaluationof parameters of the refrigerant.

In the condenser the refrigerant is present in three different phases:in a region at the refrigerant inlet, the refrigerant is in gas phaseand more or less superheated, in another region, in which therefrigerant condensates at a constant temperature, the refrigerant ispresent as a mixture of gas and liquid, and in a third region, therefrigerant is liquid and more or less sub-cooled.

In the following, t_(cond,surf)(y) is in the two-phase and liquidregions assumed to be equal to the condensation temperature of therefrigerant. For the gas phase region t_(cond,surf)(y) is assumed to bethe mean of the refrigerant gas temperature and the condensationtemperature.

For the region of the heat exchanger, where the refrigerant is presentas sub-cooled liquid, which is the region from y=0 to y=y₁, it isassumed that the temperature increases linearly with a gradient k₁,which is found to produce an adequately accurate temperature profile formost purposes. Estimating as mentioned that the condenser surfacetemperature equals the condensation temperature, the temperatureincrement can be found as:dt _(air)(y)=α(t _(liquid) +k ₁ y−t _(air)(y))dy  (3)where k₁ is a constant describing the temperature gradient in thesub-cooled region, and t_(liquid) is the refrigerant temperature at therefrigerant outlet.

For the second, two-phase region from y=y₁ to y=y₂, estimating asmentioned that the condenser surface temperature equals the condensationtemperature (t_(cond,surf)=t_(cond)),dt _(air)(y)=α(t _(cond) −t _(air)(y))dy  (4)

For the superheated gas region, i.e. the region from y=y₂ to y=y₃=1, thetemperature is assumed to vary linearly with a gradient k₂, and theequation is estimated asdt _(air)(y)=α(t _(cond) +k ₂(y−y ₂)−t _(air)(y))dy  (5)where y₂ denotes the end of the two-phase region, and k₂ denotes a meantemperature gradient for the superheated gas phase.

To obtain the air temperature in the condenser, the above equations (3),(4) and (5) are integrated, and hence:

For the sub-cooled region (0<y<y₁) $\begin{matrix}{{t_{air}\left( y_{1} \right)} = {t_{liquid} + {k_{1}y_{1}} - \frac{k_{1}}{\alpha} + {\frac{\left( {{\alpha\quad{t_{air}(0)}} + k_{1} - {\alpha\quad t_{liquid}}} \right)}{\alpha}{\mathbb{e}}^{{- \alpha}\quad y_{1}}}}} & (6)\end{matrix}$where t_(air)(0) is the air temperature at the inlet to the flow path 9,i.e. the ambient temperature.

For the two-phase region (y₁<y<y₂)t _(air)(y ₂)=t _(cond)+(t _(air)(y ₁)−t _(cond))e ^(−α(y) ² ^(−y) ¹₎  (7)For the superheated gas region (y2<y<1 (=y3)) $\begin{matrix}{{t_{air}\left( y_{3} \right)} = {t_{cond} + {k_{2}\left( {y_{3} - y_{2}} \right)} - \frac{k_{2}}{\alpha} + {\frac{\left( {{\alpha\quad{t_{air}\left( y_{2} \right)}} + k_{2} - {\alpha\quad t_{cond}}} \right)}{\alpha}{\mathbb{e}}^{- {\alpha{({y_{3} - y_{2}})}}}}}} & (8)\end{matrix}$

It is hence possible to estimate an air outlet temperature using theseequations. Parameters needed for the air outlet temperature estimate arethe air inlet temperature, the temperature of the refrigerant at theinlet and outlet, the condensation temperature of the refrigerant,estimates for y₁ and y₂, k₁ and k₂. It is found that for many condensersapproximately 5% of the heat exchange is in the first region where therefrigerant is present as sub-cooled liquid, approximately 75% of theheat exchange takes place in the second region, i.e. the part of thecondenser where the refrigerant is changing phase from gas to liquid,and the remaining approximately 20% of the heat exchange takes place inthe region of the condenser, where the refrigerant is present assuperheated gas. The value of k₁ can be established more or lessempirically based on y₁, t_(liquid) and t_(cond), whereas k₂ can beestablished more or less empirically based on y₂, the refrigerant outlettemperature, t_(cond) and the overall length of the flow path. The airoutlet temperature can thus be obtained primarily based on parameters ofthe refrigerant, and these refrigerant parameters will normally alreadybe known, as most modern refrigeration systems comprise a controller ofthe refrigeration system with sensors constantly measuring theseparameters. FIG. 4 illustrates an example of the temperature profile ofthe cross-flow condenser. If a less accurate answer is satisfactory, itis possible to use a simplified model taking e.g. only the two-phaseregion and the superheated gas region into account, or even only thetwo-phase region, where most of the heat exchange takes place.

The estimated air outlet temperature can then be compared with ameasured air outlet temperature obtained by a temperature sensor at theair outlet. When the heat exchanger experiences an abnormality, asignificant estimation error occurs, which can be used to trigger analarm signal.

Although this approach of comparing the air outlet temperature directlywith a measured temperature may be convenient and adequate in somesystems, a more stable and reliable result can be obtained when basingthe evaluation on the assumption of energy balance of the heatexchanger. However, a direct outlet temperature is rarely convenient andmoreover temperature sensor measuring air outlet temperature will seldombe present, so there is a need for an alternative approach.

The energy balance of the condenser can be stated as:{dot over (Q)}_(Air)={dot over (Q)}_(Ref)  (9)where {dot over (Q)}_(Air) is the heat taken up by the air per timeunit, i.e. the rate of heat flow delivered to the air, and {dot over(Q)}_(Ref) the heat removed from the refrigerant per time unit, i.e. therate of heat flow delivered by the refrigerant.

The basis for establishing the rate of heat flow of the refrigerant({dot over (Q)}_(Ref)) i.e. the heat delivered by the refrigerant pertime unit is the following equation:{dot over (Q)} _(Ref) ={dot over (m)} _(Ref)(h _(Ref, in) −h_(Ref, out))  (10)where {dot over (m)}_(Ref) is the refrigerant mass flow. h_(Ref,out) isthe specific enthalpy of the refrigerant at the condenser outlet, andh_(Ref,in) is the specific enthalpy of the refrigerant at the condenserinlet. The specific enthalpy of a refrigerant is a material and stateproperty of the refrigerant, and the specific enthalpy can bedetermined. The refrigerant manufacturer provides a log p, h-diagram ofthe type according to FIG. 5 for the refrigerant, wherein thethermodynamic cycle of a refrigeration system is sketched forillustration. From I to II, the refrigerant gas is compressed in acompressor, from II to III, the refrigerant is cooled in a condenserfrom a state of superheated gas to condensation and further to a stateof sub-cooled liquid. From III to IV, the refrigerant is expanded in anexpansion device to a lower pressure, where the refrigerant is presentas a mixture of liquid and gas. From IV to I, the refrigerant is heatedin an evaporator so at point I at the entry to the compressor, therefrigerant is completely gaseous.

With the aid of this diagram the specific enthalpy difference across thecondenser can be established. For example to establish h_(Ref,in) withthe aid of a log p, h-diagram, it is only necessary to know thetemperature and the pressure of the refrigerant at the condenser inlet(T_(Ref,In) and P_(cond), respectively). Those parameters may bemeasured with the aid of a temperature sensor and a pressure sensor.

Similarly, to establish the specific enthalpy at the condenser outlet,two measurement values are needed: the refrigerant temperature atcondenser outlet (T_(Ref,out)) and the pressure at the condenser outlet(P_(Cond)), which can be measured with a temperature sensor and apressure sensor, respectively.

Instead of the log p, h-diagram, it is naturally also possible to usevalues from a chart or table, which simplifies calculation with the aidof a processor. Frequently the refrigerant manufacturers also provideequations of state for the refrigerant, so a direct calculation can bemade.

The mass flow of the refrigerant may be established by assuming solelyliquid phase refrigerant at the expansion device entry. In refrigerationsystems having an electronically controlled expansion valve, e.g. usingpulse width modulation, it is possible to determine the theoreticalrefrigerant mass flow based on the opening passage and/or the openingperiod of the valve, when the difference of absolute pressure across thevalve and the subcooling (T_(V,in)) at the expansion valve entry isknown. Similarly the refrigerant mass flow can be established inrefrigeration systems using an expansion device having a well-knownopening passage e.g. fixed orifice or a capillary tube. In most systemsthe above-mentioned parameters are already known, as pressure sensorsare present, which measure the pressure in the condenser 3. In manycases the subcooling is approximately constant, small and possible toestimate, and therefore does not need to be measured. The refrigerantmass flow through the expansion valve can then be calculated by means ofa valve characteristic, the pressure differential, the subcooling andthe valve opening passage and/or valve opening period. With many pulsewidth modulated expansion valves it is found for constant subcoolingthat the theoretical refrigerant mass flow is approximately proportionalto the difference between the absolute pressures before and after andthe opening period of the valve. In this case the theoretical mass flowcan be calculated according to the following equation:{dot over (m)} _(Ref) =k _(exp)·(P _(cond) −P _(Evap))·OP  (11)where P_(Cond) is the absolute pressure in the condenser, P_(Evap) thepressure in the evaporator, OP the opening period and k_(Exp) aproportionality constant, which depend on the valve and subcooling. Insome cases the subcooling of the refrigerant is so large, that it isnecessary to measure the subcooling, as the refrigerant flow through theexpansion valve is influenced by the subcooling. In a lot of cases it ishowever only necessary to establish the absolute pressure before andafter the valve and the opening passage and/or opening period of thevalve, as the subcooling is a small and fairly constant value, andsubcooling can then be taken into consideration in a valvecharacteristic or a proportionality constant. The value of the mass flowis not critical, and another possibility is to establish the mass flowfrom the compressor directly based on empirical values e.g. datasupplied by the manufacturer of the compressor and the absolute pressurebefore and after the compressor.

Similarly the rate of heat flow heat of the air ({dot over (Q)}Air),i.e. the heat taken up by the air per time unit may be establishedaccording to the equation:{dot over (Q)} _(Air) ={dot over (m)} _(Air)(h _(Air,out) −h_(Air,in))  (12)where {dot over (m)}_(Air) is the mass flow of air per time unit,h_(Air,in) is the specific enthalpy of the air before the condenser, andh_(Air,out) is the specific enthalpy of the air after the condenser.

The specific enthalpy of the air can be calculated based on thefollowing equation:h _(Air)=1.006·t+x(2501+1.8·t),[h]=kJ/kg  (13)where t is the temperature of the air, i.e. T_(air,in) before thecondenser and T_(air,out) after the condenser. x denotes the absolutehumidity of the air. The absolute humidity of the air can be calculatedby the following equation: $\begin{matrix}{x = {0.62198 \cdot \frac{p_{w}}{p_{Amb} - p_{W}}}} & (14)\end{matrix}$

Here p_(w) is the partial pressure of the water vapour in the air, andp_(Amb) is the air pressure. p_(Amb) can either be measured or astandard atmosphere pressure can simply be used. The deviation of thereal pressure from the standard atmosphere pressure is not ofsignificant importance in the calculation of the amount of heat per timeunit delivered by the air. The partial pressure of the water vapour isdetermined by means of the relative humidity of the air and thesaturated water vapour pressure and can be calculated by means of thefollowing equation:p _(W) =p _(W,Sat) ·RH  (15)

Here RH is the relative humidity of the air and p_(W,Sat) the saturatedpressure of the water vapour. p_(W,Sat) is solely dependent on thetemperature, and can be found in thermodynamic reference books. Therelative humidity of the air can be measured or a typical value can beused in the calculation.

When equations (10) and (12) is set to be equal, as implied in equation(9), the following is found:{dot over (m)} _(Air)(h _(Air,Out) −h _(Air,In))={dot over (m)} _(Ref)(h_(Ref, In) −h _(Ref, Out))  (16)

From this the air mass flow {dot over (m)}_(Air) can be found byisolating {dot over (m)}_(Air): $\begin{matrix}{{\overset{.}{m}}_{Air} = {{\overset{.}{m}}_{Ref} \cdot \frac{\left( {h_{{Ref},{In}} - h_{{Ref},{Out}}} \right)}{\left( {h_{{Air},{Out}} - h_{{Air},{In}}} \right)}}} & (17)\end{matrix}$

Assuming faultless air flow this equation can be used to evaluate theoperation of the system. In many cases it is recommended to register theair mass flow in the system. As an example this air mass flow can beregistered as an average over a certain time period, in which therefrigeration system is running under stabile and faultless operatingconditions. Such a time period could as an example be 100 minutes. Thisestimated air mass flow found as an average under stabile and faultlessoperating conditions is denoted {dot over ({overscore (m)})}_(Air).

A certain difficulty lies in the fact that the signals from thedifferent sensors (thermometers, pressure sensors) are subject tosignificant variation. These variations can be in opposite phase, so asignal for the estimated air outlet temperature or the energy balance isachieved, which provides certain difficulties in the analysis. Thesevariations or fluctuations are a result of the dynamic conditions in therefrigeration system. It is therefore advantageous regularly, e.g. onceper minute, to establish a value, which in the following will be denoted“residual”, based on the energy balance according to equation (9):r={dot over (Q)} _(Air) −{dot over (Q)} _(Ref)  (18)so based on the equations (10) and (12), the residual can be found as:r={dot over ({overscore (m)})} _(Air)(h _(Air, Out) −h_(Air, In)−{dot over (m)}) _(Ref)(h _(Ref, In) −h _(Ref, Out))  (19)where {dot over ({overscore (m)})}_(Air) is the estimated air mass flow,which is established as mentioned above, i.e. as an average during aperiod of faultless operation. Another possibility is to assume that{dot over ({overscore (m)})}_(Air) is a constant value, which could beestablished in the very simple example of a condenser having aconstantly running fan. Even in systems with variable flow capacity,such as systems having a plurality of fans, which can be activatedindependently, or in systems incorporating one or more fans running withvariable speed, e.g. using a frequency converter, a fair estimate of themass flow can be established. The estimated mass flow can be found byestablishing the number of currently connected fans, i.e. how many fansare connected, and/or the speed of the fans, to thereby establish theflow capacity of the connected fans, e.g. by use of empirical values.

The estimated air outlet temperature can similarly by evaluated byproviding the residual as the difference between the estimated airoutlet temperature and a predetermined air outlet temperature. Thepredetermined air outlet temperature may be measured directly or may beobtained as an empirical value.

In a refrigeration system operating faultlessly, the residual r has anaverage value of zero, although it is subject to considerablevariations. To be able to early detect a fault, which shows as a trendin the residual, it is presumed that the registered value for theresidual r is subject to a Gaussian distribution about an average valueand independent whether the refrigeration system is working faultless ora fault has arisen.

In principle the residual should be zero no matter whether a fault ispresent in the system or not, as the principle of conservation of energyor energy balance of course is eternal. When it is not the case in theabove equations, it is because the prerequisite for the use of theequations used is not fulfilled in the event of a fault in the system.

In the event of fouling of the condenser surface, the thermoconductivity changes, so that a becomes several times smaller. This isnot taken into account in the calculation, so the estimated rate of heatflow of the air {dot over (Q)}_(Air) used in the equations issignificantly bigger than in reality. For the rate of heat flow of therefrigerant ({dot over (Q)}_(REF)), the calculation is correct (orassumed correct), which means that the calculated value for the rate ofheat flow of the refrigerant ({dot over (Q)}_(REF)) across the heatexchanger equals the rate of heat flow of the refrigerant in reality.The consequence is that the average of the residual becomes positive inthe event of fouling of the condenser surface.

In the event of a fault causing reduced air flow through the condenser(a defect fan or e.g. dirt covering the air inlet of the heat exchanger)the mass flow of air is less than the estimated value of the mass flowof air {dot over ({overscore (m)})}_(Air) used in the calculations. Thismeans that the rate of heat flow of the air used in the calculations islarger than the actual rate of heat flow of the air in reality, i.e.less heat per unit time is removed by the air than expected. Theconsequence (assuming correct rate of heat flow of the refrigerant), isthat the residual becomes positive in the event of a fault causingreduced air flow across the condenser.

To filter the residual signal for any fluctuations and oscillationsstatistical operations are performed by investigating the residual.

The investigation is performed by calculating an abnormality indicatoraccording to the following equation: $\begin{matrix}{S_{\mu,i} = \left\{ \begin{matrix}{{S_{\mu,{i - 1}} + s_{i}},{{{{when}\quad S_{\mu,{i - 1}}} + s_{\mu,i}} > 0}} \\{0,{{{{when}\quad S_{\mu,{i - 1}}} + s_{\mu,i}} \leq 0}}\end{matrix} \right.} & (20)\end{matrix}$where s_(μ,i) is calculated according to the following equation:$\begin{matrix}{s_{\mu,i} = {c_{1}\left( {r_{i} - \frac{\mu_{0} + \mu}{2}} \right)}} & (21)\end{matrix}$where c₁ is a proportionality constant, μ₀ a first sensibility value, μa second sensibility value, which is positive.

In equation (20) it is naturally presupposed that the abnormalityindicator S_(μ,i), i.e. at the first point in time, is set to zero. Fora later point in time is used s_(μ,i) according to equation (21), andthe sum of this value and the abnormality indicator S_(μ,i) at aprevious point in time is computed. When this sum is larger than zero,the abnormality indicator is set to this new value. When this sum equalsor is less than zero, the abnormality indicator is set to zero. In thesimplest case μ₀ is set to zero. μ is a chosen value, which e.g.establish that a fault has arisen. The parameter μ is a criterion forhow often it is accepted to have a false alarm regarding heat exchangerabnormality detection.

When for example a fault occurs in that the air inlet of the condenseris covered by e.g. leaves, then the abnormality indicator will grow, asthe periodically registered values of the s_(μ,i) in average is largerthan zero. When the abnormality indicator reaches a predetermined valuean alarm is activated, the alarm showing that the air mass flow isreduced. If a larger value of μ is chosen, fewer false alarms areexperienced, but there exist a risk of reducing sensitivity fordetection of a fault.

The principle of operation of the filtering according to equation (20)and (21) shall be illustrated by means of FIGS. 7 and 8, where thefiltering is used on the residual found using energy balance, i.e. basedon equation (18). In FIG. 7 the time in minutes is on the x-axis and onthe y-axis the residual r. FIG. 7 illustrates the emerging of a fault inthat the condenser of a shop was subject to a sudden fouling atapproximately t=2900 minutes. However, as can be seen the signal issubject to quite significant fluctuations and variations, which makesevaluation difficult, and the presence of a problem is really notevident before approximately t=5500 minutes.

In FIG. 8, which represent the filtering of the data of FIG. 7 withmeans of the abnormality indicator according to equation (20), the timein minutes in on the x-axis and on the y-axis the abnormality indicatorS. As can be seen, the heat exchanger was working properly untilapproximately t=2900 minutes, when a sudden fouling took place, and theabnormality indicator S rose. This is easier to see in FIG. 9, which isan enlarged portion of FIG. 8. In FIG. 9 the abnormality atapproximately t=2900 minutes can be easily detected using theabnormality indicator S compared to using the residual or the air outlettemperature.

A further advantage of the device is that it may be retrofitted to anyrefrigeration or heat pump system without any major intervention in therefrigeration system. The device uses signals from sensors, which arenormally already present in the refrigeration system, or sensors, whichcan be retrofitted at a very low price.

While the present invention has been illustrated and described withrespect to a particular embodiment thereof, it should be appreciated bythose of ordinary skill in the art that various modifications to thisinvention may be made without departing from the spirit and scope of thepresent invention.

1. A method for detecting an abnormality of a heat exchanger exchangingheat between a first fluid flow flowing in a conduit and a second fluidflow flowing along a flow path, said conduit and said flow path eachhaving an inlet and an outlet, said method comprising the steps of:establishing at least one parameter representative of the temperatureconditions of the heat exchanger; establishing a second fluid inlettemperature; establishing a parameter indicative of expected heatexchange between the heat exchanger and the second fluid; establishingan estimated second fluid outlet temperature; and employing theestimated second fluid outlet temperature for evaluating the heatexchange between the first and second fluids by comparing the estimatedsecond fluid outlet temperature, or a parameter derived therefrom, witha reference value, wherein the estimated second fluid outlet temperatureis established from at least one parameter representative of thetemperature conditions of the head exchanger, the second fluid inlettemperature and the parameter being indicative of an expected heatexchange.
 2. The method according to claim 1, wherein the referencevalue is a predetermined second fluid outlet temperature.
 3. The methodaccording to claim 1, wherein the estimated second fluid outlettemperature is used for establishing a second heat rate of the secondfluid for evaluating the energy balance of the second heat rate of thesecond fluid compared to a first heat rate of the first fluid.
 4. Themethod according to claim 3, wherein the second rate of heat flow of thesecond fluid is established by establishing an estimate of a secondfluid mass flow and a specific enthalpy change of the second fluidacross the heat exchanger based on the estimated second fluid outlettemperature and the second fluid inlet temperature, and the condensationpressure.
 5. The method according to claim 3, wherein the first rate ofheat flow is established by establishing a first fluid mass flow and aspecific enthalpy change of the first fluid across the heat exchangerbased on parameters representative for first fluid inlet and outlettemperatures.
 6. The method according to claim 3, wherein a residual isestablished as difference between the first heat rate and the secondheat rate.
 7. The method according to claim 2, wherein a residual isestablished as difference between the estimated and predetermined secondfluid outlet temperature.
 8. The method according to claim 6, wherein anabnormality indicator is provided by means of the residual, theabnormality indicator being provided according to the formula:$\begin{matrix}{S_{\mu,i} = \left\{ \begin{matrix}{{S_{\mu,{i - 1}} + s_{i}},{{{{when}\quad S_{\mu,{i - 1}}} + s_{\mu,i}} > 0}} \\{0,{{{{when}\quad S_{\mu,{i - 1}}} + s_{\mu,i}} \leq 0}}\end{matrix} \right.} & (20)\end{matrix}$ where s_(μ,i) is calculated according to the followingequation: $\begin{matrix}{s_{\mu,i} = {c_{1}\left( {r_{i} - \frac{\mu_{0} + \mu}{2}} \right)}} & (21)\end{matrix}$ where r_(i): residual c₁: proportionality constant μ₀:first sensibility value μ: second sensibility value.
 9. A heat exchangerabnormality detection device for a heat exchanger exchanging heatbetween a first fluid in a conduit and a second fluid in a flow path thedevice comprising: a first estimator estimating at least one parameterrepresentative of the temperature conditions of the heat exchanger; afirst intermediate memory means storing at least one parameterrepresentative of the temperature conditions of the heat exchanger; atemperature sensor measuring the second fluid inlet temperature; asecond intermediate memory means storing the second fluid inlettemperature, a second estimator establishing a parameter indicative ofexpected heat exchange between the heat exchanger and the second fluid;a third intermediate memory means storing the parameter indicative ofexpected heat exchange; a processor establishing an estimated secondfluid outlet temperature; and a comparator comparing the estimatedsecond fluid outlet temperature, or a parameter established on basisthereof, with a reference value; wherein the estimated second fluidoutlet temperature is based on said at least one parameterrepresentative of the temperature conditions of the heat exchanger, saidsecond fluid inlet temperature, from the first and second intermediatememory means, respectively, and the parameter indicative of expectedheat exchange from the third intermediate memory means.
 10. Thedetection device according to claim 9, wherein the detection devicefurther comprises memory means for storing at least one parameter fromthe processor.
 11. The detection device according to claim 9, whereinthe heat exchanger is part of a vapour-compression refrigeration or heatpump system comprising a compressor, a condenser, an expansion device,and an evaporator interconnected by conduits providing a flow circuitfor the first fluid, said first fluid being a refrigerant.
 12. Thedetection device according to claim 11, wherein the heat exchanger isthe condenser.
 13. The detection device according to claim 9, whereinthe second fluid is air.
 14. The detection device according to claim 11,wherein that the condenser is part of a refrigerated display cabinetpositioned within a building and the condenser is positioned outside thebuilding.
 15. The detection device according to claim 9, wherein thedetection device is used for detecting fouling of the heat exchangerand/or detecting insufficient flow of the second fluid.